The present invention relates generally to the detection of manufacturing defects in composite insulators and, more particularly, to an apparatus and method for detecting defects in composite insulators used in power systems.
Composite insulators consist of a fiberglass rod with two metal end fittings attached. The rod is then coated with rubber material protecting the rod from the environment. It is possible that during the manufacturing process the rod is partially damaged and may result in a failure in the field (may happen many years after installation). The damage may occur while either the end fittings are being attached to the rod (usually by crimping), while the rubber material is being molded on, or during handling (especially when the rod is hot after molding).
For a number of years, investigations of vibration-based field inspection methods have been conducted to identify damaged insulators (both NCI and porcelain−note NCI=non-ceramic insulator=composite insulator=polymer insulator). After several investigations were performed on composite insulators, it was determined that while insulator damage could be detected in a vibration response, a number of variables, such as tension load and attachments, could affect the vibration response more strongly than the damage. This has made the technique challenging, almost impossible, to apply in field situations. For this reason, vibration-based inspection is best suited for a laboratory-type environment rather than a field installation.
Currently, two types of tests are used in the manufacturing process to identify potential rod defects: (1) To listen to acoustic signals while crimping the end fitting on to the rod—if the rod cracks or is over crimped a specific signal is identified, and (2) perform a Routine Test Load (RTL) test.
The RTL is performed at the end of the manufacturing process. All units are required to be subjected to an RTL which is 50% of their specified mechanical Load. This is a proper time to conduct the RTL since the geometry is well defined as is the mechanical load. However, while the RTL test provides a basis for mechanical strength, it has deficiencies in detecting defects in composite insulators.